Implement wxSlider ticks support in wxGTK

Tick marks were not available for wxSlider under GTK+ 2 or GTK+ 3
implementations of wxWidgets. However, tick marks have been available
for the GtkScale widget since GTK+ 2 version 2.16.

This change adds similar functionality in relation to tick marks
as was already available in wxMSW builds.

Closes https://github.com/wxWidgets/wxWidgets/pull/1355

include/wx/gtk/slider.h

@@ -56,6 +56,10 @@ public:
     virtual void SetThumbLength(int lenPixels) wxOVERRIDE;
     virtual int GetThumbLength() const wxOVERRIDE;
 
+    virtual void ClearTicks() wxOVERRIDE;
+    virtual void SetTick(int tickPos) wxOVERRIDE;
+    int GetTickFreq() const wxOVERRIDE;
+
     static wxVisualAttributes
     GetClassDefaultAttributes(wxWindowVariant variant = wxWINDOW_VARIANT_NORMAL);
 
@@ -73,11 +77,20 @@ protected:
     GtkWidget *m_minLabel,*m_maxLabel;
     bool m_blockScrollEvent;
 
+    // Note the following member is not used in GTK+2 < 2.16.
+    int m_tickFreq;
+
     virtual GdkWindow *GTKGetWindow(wxArrayGdkWindows& windows) const wxOVERRIDE;
 
     // set the slider value unconditionally
     void GTKSetValue(int value);
 
+    // Platform-specific implementation of SetTickFreq
+    virtual void DoSetTickFreq(int freq) wxOVERRIDE;
+
+private:
+    void Init();
+
     wxDECLARE_DYNAMIC_CLASS(wxSlider);
 };